Fusion formed and ion exchanged glass-ceramics

ABSTRACT

The present disclosure relates to fusion formable highly crystalline glass-ceramic articles whose composition lies within the SiO 2 —R 2 O 3 —Li 2 O/Na 2 O—TiO 2  system and which contain a silicate crystalline phase comprised of lithium aluminosilicate (β-spodumene and/or β-quartz solid solution) lithium metasilicate and/or lithium disilicate. Additionally, these silicate-crystal containing glass-ceramics can exhibit varying Na 2 O to Li 2 O molar ratio extending from the surface to the bulk of the glass article, particularly a decreasing Li 2 O concentration and an increasing Na 2 O concentration from surface to bulk. According to a second embodiment, disclosed herein is a method for forming a silicate crystalline phase-containing glass ceramic.

This application is a divisional application of and claims the benefitof priority under 35 U.S.C. §120 of U.S. patent application Ser. No.13/306,385 on Nov. 29, 2011, now U.S. Pat. No. 8,883,663 which in turnclaims the benefit of priority under 35 U.S.C. §119 of U.S. ProvisionalApplication Ser. No. 61/418,097 filed on Nov. 30, 2010, the content ofwhich is relied upon and incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present disclosure herein relates generally to glass-ceramics, inparticular to fusion formed and ion exchanged lithium aluminosilicateglass-ceramic glass article comprising β-spodumene and/or β-quartz solidsolution.

BACKGROUND

Glass-ceramics are polycrystalline materials formed by a controlledcrystallization of a precursor glass. In general, the method forproducing such glass-ceramics customarily involves three fundamentalsteps: first, melting a glass-forming batch containing the selectedmetallic oxides; second, cooling the melt to a temperature at leastbelow its transformation range, while simultaneously forming a glassbody of a desired geometry; and third, heating the glass body to atemperature above the transformation range of the glass in a controlledmanner to generate crystals in situ. To develop nuclei in the glass, theglass will be heated initially to a temperature within or somewhat abovethe transformation range for a period of time; although there arecertain compositions that are known to be self-nucleating and thus donot require the development of nuclei. Thereafter, the temperature willbe raised to temperatures where crystals can grow from the nuclei. Theresulting crystals are typically uniformly distributed and fine-grained.Internal nucleation permits glass-ceramics to have favorable qualitiessuch as a very narrow distribution of particle size and a highly uniformdispersion of crystals throughout the glass host.

Glass-ceramics have not been formed by the fusion process because thisprocess requires a much higher viscosity at the liquidus than thatavailable in the precursor glasses of glass-ceramics. Depending uponparticular compositions and the forming parameters implemented, thefusion process requires viscosities at the liquidus of at least 75,000poise, in some cases, of well over 100,000 poises, and more typicallyabove 500,000 poises. The parent glasses of glass-ceramics, designed tocrystallize easily, typically have viscosities at their liquidi of10,000 poises or below, and to our knowledge never above 20,000 poises.They therefore are not amenable to fusion forming. This presents aproblem and an opportunity, because glass-ceramics offer desirableproperties not achievable in fusion formable glasses. These propertiesinclude opacity, various degrees of translucency and surface luster,pastel colors, and perhaps most importantly, a low or essentially zerocoefficient of thermal expansion. Thus glass-ceramics have a widervariety of aesthetic appearances as well as heat and fire resistance.Moreover, glass-ceramics are generally stronger than glass, and throughsurface ion exchange processing can often be made stronger thanion-exchanged glass because of lower stress relaxation at salt-bathtemperatures.

Down draw processing of glass, particularly the fusion processing ofglass exhibits the inherent advantages of the formation of a resultantpristine surface and the ability to product glass articles (e.g.,sheets) exhibiting thinness dimension on the order to 2 mm or less.

As such, it would be desirable to identify glass-ceramics which can bemade from fusion formed glasses thus resulting in the formation of thinglass-ceramic articles exhibiting pristine surfaces and exhibiting theintrinsic benefits of glass-ceramics (when compared to glasses), namelystrength, low CTE, and associated thermal shock resistance, andcolor/opacity variation.

SUMMARY

Disclosed herein are fusion formable highly crystalline glass-ceramicarticles whose composition lies within the SiO₂—R₂O₃—Li₂O/Na₂O—TiO₂system and which contain a silicate crystalline phase comprised oflithium aluminosilicate, specifically β-spodumene and/or β-quartz solidsolution, as well as, lithium metasilicate and/or lithium disilicate.Additionally, these silicate glass-ceramics can exhibit varying Na₂O toLi₂O molar ratio extending from the surface to the bulk of the glassarticle, particularly a decreasing Li₂O concentration and an increasingNa₂O concentration from surface to center.

In one embodiment these translucent or opaque silicatecrystal-containing glass-ceramics comprise, in weight percent on anoxide basis, of 40-80% SiO₂, 2-30% Al₂O₃, 2-10% Li₂O, 0-8% TiO₂, 0-3%ZrO₂, 0-2% SnO₂, 0-7% B₂O₃, 0-4% MgO, 0-12% ZnO, 0-8% BaO, 0-3% CaO,0-6% SrO, 0-4% K₂O, up to 2% Na₂O, 0-1.0% Sb₂O₃, 0-0.25% Ag, 0-0.25CeO₂, the molar ratio of Li₂O+Na₂O/Al₂O₃+B₂O₃ greater than 0.8, and thecombination of TiO₂+ZrO₂+SnO₂ in an amount of at least 3.0 mol %.

According to a second embodiment, disclosed herein is a method forforming a silicate crystal-containing glass ceramic which comprises thefollowing steps: (a) melting a batch for, and down drawing a glassarticle having a composition comprising, in weight percent on an oxidebasis, of 40-80% SiO₂, 2-30% Al₂O₃, 5-30% Na₂O, 0-8% TiO₂, 0-12% ZrO₂,0-2% SnO₂, 0-7% B₂O₃, 0-4% MgO, 0-6% ZnO, 0-8% BaO, 0-3% CaO 0-3, 0-6%SrO 0-6, 0-4% K₂O, 0-2% Li₂O, 0-1.0% Sb₂O₃, 0-0.25% Ag, 0-0.25 CeO₂, themolar ratio of Na₂O/Al₂O₃+B₂O₃ of greater than 0.8, and the combinationof TiO₂+ZrO₂+SnO₂ in an amount of at least 3.0 mol %; (b.) ionexchanging the glass article by placing the glass article in aLi-containing salt bath exhibiting a temperature sufficiently above theglass strain point and holding the glass sheet for time sufficient tocomplete ion exchange of Li for Na ions substantially throughout theglass article; (c.) ceramming the glass to a temperature between about550-1100° C. for a period of time sufficient to cause the generation ofa glass-ceramic which contains a predominant silicate crystal phase oflithium aluminosilicate (β-spodumene and/or β-quartz solid solution),lithium metasilicate and/or lithium disilicate phase and exhibits aglass-ceramic composition within the SiO₂—R₂O₃—Li₂O/Na₂O—TiO₂ system;and (d.) cooling the glass-ceramic article to room temperature.

In a further embodiment the ion-exchanging and the ceramming steps forthe glass article can be accomplished simultaneously and are performedin the Li-containing salt bath exhibiting a temperature sufficientlyabove the glass strain point.

Additional features and advantages of the embodiments described hereinwill be set forth in the detailed description which follows, and inpart, will be readily apparent from that description or recognized bypracticing the embodiments described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description describe various embodiments and areintended to provide an overview or framework for understanding thenature and character of the claimed subject matter. The accompanyingdrawings are included to provide a further understanding of the variousembodiments, and are incorporated into and constitute a part of thisspecification. The drawings illustrate the various embodiments describedherein, and together with the description serve to explain theprinciples and operations of the claimed subject matter.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a plot illustrating the respective Li₂O and Na₂Osurface-to-depth concentrations of an illustrative embodiment disclosedherein wherein the glass/glass-ceramic article has been IOX'd to producean alkali gradient;

FIG. 2 is a plot illustrating the respective Li₂O and Na₂Osurface-to-depth concentrations of an illustrative embodiment disclosedherein wherein the glass/glass-ceramic article has been IOX'd such thatsubstantially all the Na₂O has been replaced by Li₂O, thus no alkaligradient.

FIG. 3 is an SEM micrograph illustrating the Li-metasilicate crystalstructure of an illustrative embodiment disclosed herein.

DETAILED DESCRIPTION

In the following description, whenever a group is described ascomprising at least one of a group of elements and combinations thereof,it is understood that the group may comprise, consist essentially of, orconsist of any number of those elements recited, either individually orin combination with each other. Similarly, whenever a group is describedas consisting of at least one of a group of elements or combinationsthereof, it is understood that the group may consist of any number ofthose elements recited, either individually or in combination with eachother. Unless otherwise specified, a range of values, when recited,includes both the upper and lower limits of the range as well as anysub-ranges therebetween.

The present disclosure is based on the discovery of a family of fusionformable glass compositions that can produce glasses of excellentstability which can be ion exchanged (whereby Li replaces some portionof the Na ions) and thereafter cerammed to produce substantiallytranslucent or opaque glass-ceramics having silicate as the predominantcrystal phase with that silicate crystal phase being a lithiumaluminosilicate phase β-spodumene and/or β-quartz solid solution phase),a lithium disilicate and/or lithium metasilicate crystal phase.

The current disclosure addresses the currently recognized inability tofusion draw glass-ceramic articles due to the typical low viscositiesassociated with precursor glasses used to form glass ceramics; i.e., theliquidus viscosities typically exhibited by precursor glasses for glassceramics are in the 10,000-20,000 poises range which is far below theminimum 75,000 poise required for certain down-draw processes such assome fusion processes. In other words, inherently glasses which canreadily be crystallized to form glass-ceramics have typically have low arelatively viscosity at the liquidus, generally less than theaforementioned 10,000-20,000 poise and thus do not lend themselves tofusion drawability which requires the viscosity of the liquidus to be atleast higher than around 75,000 poise, depending upon the compositionand the forming conditions utilized. Generally, the inventors haveessentially solved this low liquidus viscosity problem exhibited byglass ceramic precursor glasses by fusion-forming a glass compositionwith high viscosity at the liquidus and subsequently converting thisglass, by high temperature Li for the Na ion exchange (complete ornearly complete ion exchange) at a temperature well above the strainpoint, to a glass corresponding to the original precursor glasscomposition for the desired glass-ceramic. This new glass, with most, ifnot all of the Na replaced by Li, can then be converted to the requisiteglass-ceramic by standard heat treatment processing which achievescrystallization and the formation of the glass-ceramic.

Recognition and solution of the aforementioned problem has resulted inthe discovery of fusion formable highly crystalline glass-ceramicarticles whose composition lies within the SiO₂—R₂O₃—Li₂O/Na₂O—TiO₂system. These glass ceramic materials contain a silicate crystallinephase comprised of a lithium aluminosilicate phase β-spodumene and/orβ-quartz solid solution), lithium disilicate and/or lithium metasilicatecrystal phase.

Disclosed herein is a substantially translucent or opaque, lithiumaluminosilicate, disilicate, or metasilicate crystal-containingglass-ceramics which exhibits a base composition comprising, in weightpercent on the oxide basis, of the constituents listed in Table I:

TABLE I SiO₂ 40-80% Al₂O₃  2-30% Li₂O  2-10% TiO₂ 0-8  ZrO₂, 0-3  SnO₂0-2% B₂O₃ 0-7% MgO 0-7% ZnO  0-12% BaO, 0-8% CaO 0-3% SrO 0-6% K₂O 0-4%CeO2  0-0.02 Ag2O  0-0.1% Na₂O up to 3% Au   0-0.01% Sb₂O₃  0-1.0%Li₂O + Na₂O/ ≧0.8 Al₂O₃ + B₂O₃

Additionally, these silicate crystal containing glass-ceramics canexhibit varying Na₂O to Li₂O molar ratio, or molar ratio gradient,extending from the surface into the bulk of the glass article. Inparticular, this molar ratio variation is exhibited as high Li₂Oconcentration on the surface which decreases into the bulk and low Na₂Oconcentration exhibited on the surface which increases into the surface.Furthermore, this Li₂O to Na₂O gradient (surface to bulk) results in ahigher level of Li-crystallinity being exhibited on the surface, andthus lower expansion when compared the bulk which exhibits a loweredlevel of Li-crystallinity (higher Na-crystallinity) and higherassociated expansion; i.e., since the Li-aluminosilicate crystals havelower thermal expansion when compared to the Na-aluminosilicate crystalsit follows that the surface would exhibit a lower thermal expansion.This difference in expansion or expansion mismatch (low on the surfaceand higher in the bulk) results in a silicate-crystal containingglass-ceramic which exhibits the added benefit/attribute of some levelof compression on the surface and thus a glass-ceramic article whichinherently exhibits added strength a surface which is more damage andscratch resistant.

Colorants in the form of metallic ions may be present in order to impartvarious colors or tints to the glass. Specifically, those metallic ionswhich can achieve this colorant feature include those transition metalions selected from the group consisting of Co²⁺, Cr³⁺, Cu¹⁺, Sn⁴⁺, Mn⁴⁺,Sb³⁺, Fe³⁺, In³⁺, Bi³⁺, Ni²⁺, V³⁺, and Ta⁵⁺.

Maintaining an Li₂O+Na₂O/Al₂O₃+B₂O₃ molar ratio of greater than 0.8 isnecessary to avoid the formation of high liquidus phases such as mulliteand to result in the production of desired silicate crystal phases suchas, but not limited to, lithium aluminosilicate (β-spodumene and/orβ-quartz solid solution), lithium metasilicate and/or lithiumdisilicate.

It should be noted that in certain embodiments, particularly thoselithium aluminosilicate (β-spodumene and/or β-quartz solid solution)containing glass-ceramics, a further compositional requirement ofTiO₂+ZrO₂+SnO₂ amounts in excess 3.0 wt % is required in the compositionand functions as a nucleation package. In other words, this total amountof nucleating agents are required in the glass-ceramic (precursor glasscomposition) so that effective nucleation is initiated and the necessarycrystal growth is achieved. Although it should be noted that TiO₂amounts exceeding 4.5% should be avoided due to the resultant highrutile liquidus which has the potential to increase problems in theinitial fusion forming of the glass.

According to another embodiment the base glass-ceramic compositioncomprises, in weight percent on the oxide basis, of the followingconstituents: 55-70% SiO₂, 17-23% Al₂O₃, 0-5% B₂O₃, 2.5-7% Li₂O, 0-3%ZrO₂, 0-1% SnO₂, 0-3% MgO, 0-6% ZnO, 0-4% Na₂O, and 2-4.5% TiO₂.

In another embodiment, particularly those glass ceramics where thesilicate crysal phase comprises either lithium metasilicate and/orlithium disilicate, one representative base glass-ceramic compositioncomprises, in weight percent, 74-81% SiO₂, 4.5-10% Al₂O₃, 8.8-10.1%Li₂O, greater than zero to less than 0.3% CeO₂, 1.5-1.7% ZnO, 1.2-1.6%Na₂O, 2.2-3.8% K₂O, >0 to 0.25% Sb₂O₃, less than 0.1% SnO₂, and fromgreater than zero up to 0.3% of at least one metal selected from thegroup of gold (Au) and silver (Ag), or mixtures thereof, provided thatthe sum of Au+Ag is not greater than 0.3%.

The following disclosure relates to a method for forming a lithiumaluminosilicate, disilicate and/or metasilicate glass ceramics. In itsmost general form the method comprises the following steps: (a.) meltinga batch for, and down drawing a glass article having a compositioncomprising, in weight percent on an oxide basis, of 40-80% SiO₂, 2-30%Al₂O₃, 5-30% Na₂O, 0-8% TiO₂, 0-3% ZrO₂, 0-2% SnO₂, 0-7% B₂O₃, 0-7% MgO,0-12% ZnO, 0-8% BaO, 0-3% CaO 0-3, 0-6% SrO 0-6, 0-4% K₂O, 0-3% Li₂O,0-1.0% Sb₂O₃, 0-0.25% Ag, 0-0.25% CeO₂, 0-0.01% Au, a molar ratio ofLi2O+Na₂O/Al₂O₃+B₂O₃ of greater than 0.8%, wherein the batched glassexhibits a liquidus viscosity of greater than 75,000 in the down drawingof the glass article; (b.) ion exchanging the glass article by placingthe glass article in a Li-containing salt bath exhibiting a temperaturesufficiently above the glass strain point and holding the glass sheetfor time sufficient to complete ion exchange of Li for Na ionssubstantially throughout the glass article; (c.) ceramming the glass toa temperature between about 550-1100° C. for a period of time sufficientto cause the generation of a glass-ceramic which contains a predominantsilicate crystal phase of lithium aluminosilicate (β-spodumene and/orβ-quartz solid solution), lithium metasilicate and/or lithiumdisilicate, and exhibits a glass-ceramic composition within theSiO₂—R₂O₃—Li₂O/Na₂O—TiO₂ system; and (d.) cooling the glass-ceramicarticle to room temperature.

In another embodiment the method involves utilizing a batch compositionconsisting essentially, in weight percent on an oxide basis, of 55-70%SiO₂, 17-23% Al₂O₃, 0-5% B₂O₃, 10-20% Na₂O, 0-2% Li₂O, 0-3% ZrO₂, 0-1%SnO₂, 0-2% MgO, 0-3% ZnO, and 2-4.5% TiO₂.

The glass batch composition further include a transition metal ionselected from the group consisting of Co²⁺, Cr³⁺, Cu¹⁺, Sn⁴⁺, Mn⁴⁺,Sb³⁺, Fe³⁺, In³⁺, Bi³⁺, Ni²⁺, V³⁺, Ta⁵⁺.

Inclusion of small amounts of Li₂O in the batch of the precursor glasshave the potential to reduce the time necessary to complete ion exchangeprocess due to the fact that less Li for Na ion exchange will be neededto achieve the desired level of Li in the final glass-ceramic. However,it is recommended that no more than 3 wt. % Li₂O (approximately ¼ offinal desired glass-ceramic Li₂O amount) be included in the precursorglass batch. Li₂O levels exceeding 2 wt. % result in rapidcrystallization conditions which tend to produce a higher liquidus dueto the formation of a β-spodumene phase, which detrimentally effects theability to down draw (fusion form) glass articles.

Regarding the down draw process it is contemplated that this includeseither a fusion draw glass process or a slot draw process.

The fusion draw process uses a drawing tank that has a channel foraccepting molten glass raw material. The channel has weirs that are openat the top along the length of the channel on both sides of the channel.When the channel fills with molten material, the molten glass overflowsthe weirs. Due to gravity, the molten glass flows down the outsidesurfaces of the drawing tank. These outside surfaces extend down andinwardly so that they join at an edge below the drawing tank. The twoflowing glass surfaces join at this edge to fuse and form a singleflowing sheet. The fusion draw method offers the advantage that, sincethe two glass films flowing over the channel fuse together, neitheroutside surface of the resulting glass sheet comes in contact with anypart of the apparatus. Thus, the surface properties are not affected bysuch contact.

The slot draw method is distinct from the fusion draw method. Here themolten raw material glass is provided to a drawing tank. The bottom ofthe drawing tank has an open slot with a nozzle that extends the lengthof the slot. The molten glass flows through the slot/nozzle and is drawndownward as a continuous sheet therethrough and into an annealingregion. Compared to the fusion draw process, the slot draw processprovides a thinner sheet, as only a single sheet is drawn through theslot, rather than two sheets being fused together, as in the fusiondown-draw process.

As used herein, the term “ion-exchanged” is understood to mean treatingthe heated aluminosilicate precursor glass with a heated solutioncontaining ions having a different ionic radius than ions that arepresent in the glass surface and/or bulk, thus replacing those ions withsmaller ions with the larger ions or vice versa depending on the ionexchange temperature conditions. Potassium ions, for example, couldeither replace, or be replaced by, sodium ions in the glass, againdepending upon the ion exchange temperature conditions. Alternatively,other alkali metal ions having larger atomic radii, such as rubidium orcesium could replace smaller alkali metal ions in the glass. Similarly,other alkali metal salts such as, but not limited to, sulfates, halides,and the like may be used in the ion exchange process.

In the instant method, it is contemplated that both types of ionexchange can take place; i.e., larger for smaller ions are replaced andsmaller for larger ions are replaced. In one embodiment, the methodinvolves ion exchanging the glass article by placing the glass articlein a Li-containing salt bath, exhibiting a temperature sufficientlyabove the glass strain point, and holding the glass sheet for timesufficient to complete ion exchange of Li for Na ions substantiallythroughout the glass article. In some embodiments this IOX process takesplace at temperatures above 500° C., particularly for those precursorglasses where the crystal phase comprises lithium disilicate and/orlithium metasilicate. In other embodiments the IOX process takes placeabove 700° C., particularly for those precursor glasses where thecrystal phase comprises lithium aluminosilicate (β-spodumene and/orβ-quartz solid solution). In certain embodiments (lithiumaluminosilicate) the molten salt bath is a high temperature sulfate saltbath composed of Li₂SO₄ as a major ingredient, but diluted with Na₂SO₄,K₂SO₄ or Cs₂SO₄ in sufficient concentration to create a molten bath.Alternatively, in those lithium disilicate and/or lithium metasilicateembodiments, the baths may be comprised of pure LiNO₃ as the majorexchange salt. This ion-exchange step functions to replace the largersodium ions in the glass structure with the smaller lithium ions whichare found in the Li-containing salt bath. Regardless of the embodiment,the precursor Na-containing glass which has been fusion drawn istransformed into precursor Li-containing glass which can be cerammed toproduce a glass-ceramic having a predominant silicate crystal phase,either a lithium aluminosilicate crystal phase (β-spodumene and/orβ-quartz solid solution) or alternatively a phase comprising eitherlithium-metasilicate or lithium disilicate. It should be noted that thision exchange is accomplished a temperature sufficiently above the glassstrain point so as to relieve any tension which would be created in theglass article as a result of the replacing of a larger Na ions withsmaller Li ions and thus avoiding the creation any undesiredmicrocracking.

It is contemplated that the aforementioned varying of Na₂O to Li₂O molarratio, or molar ratio gradient, extending from the surface into the bulkof the glass article can be controlled or achieved through the IOXprocess conditions utilized. Longer IOX process times or higher IOXtemperatures are necessary for achieving complete IOX where all of theNa is exchanged for the Li; i.e., no gradient and a flat alkali profile.One skilled in the art will be able to determine the time andtemperature necessary to achieve the gradient or non gradient alkaliprofiles.

In a further embodiment the ion-exchanging and the ceramming steps forthe glass article can be accomplished simultaneously and can beperformed in the Li-containing salt bath exhibiting a temperaturesufficiently above the glass strain point. In order to achievesimultaneous ion-exchange and ceramming it is recommended that bathtemperatures exceeding 800° C. be utilized. As described above for theion-exchange alone step, the molten salt bath is a high temperaturesulfate salt bath composed of Li₂SO₄ as a major ingredient, but dilutedwith Na₂SO₄, K₂SO₄ or Cs₂SO₄ in sufficient concentration to create amolten bath.

The advantages of this combined ion-exchanging/ceramming technique overthe process where ceramming is subsequent to the ion-exchange stepinclude: (1) allowing the glass to relax sufficiently in the bath sothat tensile stresses from Li+ for Na+ exchange are dissipated, (2)reduced handling of the glass/glass-ceramic article and thus reducedopportunities for damaging the glass/glass-ceramic articles, and in turnreduced costs; (3) ceramming thin articles like sheet in a salt bathwhere the effective weight of the glass is greatly reduced by buoyancy,allows maintenance of a pristine surface, especially in comparison tonormal atmosphere ceramming where contact with a substrate can blemishthe surface of the article. It should be noted that one additionalbenefit of the combined step is that higher salt bath temperatures whichachieve the ceramming to form the glass-ceramic are lower than thosetemperatures required in normal atmosphere ceramming of the same type ofglass.

In a still further embodiment the so-formed glass ceramic article can besubject a post-ceramming ion-exchange process. In this secondion-exchange step the glass-ceramic article is placed, after ceramming,in a second Na-containing ion-exchange bath at a temperature below theglass strain point. The glass-ceramic is held in the ion exchange bathfor a sufficient time for ion exchange to occur on the surface and intosome depth into the article. Particularly the second ion-exchangeprocess results in the exchange small Li for large Na ions thus creatinga compressive surface layer. In other words, surface compressive stressis created to a stress caused by the substitution during chemicalstrengthening of an alkali metal ion contained in a glass surface layerby an alkali metal ion having a larger ionic radius. In this embodiment,the down-drawn glass is chemically strengthened by placing it a moltensalt bath comprising NaNO₃ for a predetermined time period to achieveion exchange. In one embodiment, the temperature of the molten salt bathis about 430° C. and the predetermined time period is about eight hours.

The replacement of smaller ions by larger ions at a temperature belowthat at which the glass network can relax produces a distribution ofions across the surface of the glass that results in a stress profile.The larger volume of the incoming ion produces compressive stress (CS)on the surface and tension in the center (CT) of the glass. Thecompressive stress is related to the central tension by the followingrelationship:CS=CT×(t−2DOL)/DOL;where t is the thickness of the glass and DOL is the depth of exchange.

EXAMPLES

The following examples illustrate the advantages and features of theinvention and in are no way intended to limit the invention thereto:

Inasmuch as the sum of the individual constituents totals or veryclosely approximates 100, for all practical purposes the reported valuesmay be deemed to represent mole percent. The actual batch ingredientsmay comprise any materials, either oxides, or other compounds, which,when melted together with the other batch components, will be convertedinto the desired oxide in the proper proportions.

Example 1

In a first example, a precursor glass from the basic SiO₂—Al₂O₃—Na₂Osystem and which was capable of being down-drawn (particularly fusionformed) into a glass article/sheet was formed. Specifically, a simplesodium aluminosilicate glass with the following batched composition, inweight percent, was produced: 56.6% SiO₂, 24.0% Al₂O₃, 14.6% Na₂O and4.8% TiO₂; Titania was present at levels required for eventualnucleation. This glass was batched, mixed and melted in a platinumcrucible at 1650° C. and thereafter annealed at 650° C. It was expectedthat glass would yield a low expansion lithium aluminosilicateglass-ceramic after Li+ for Na+ exchange and subsequent heat treatment.

Glass squares about 1″ in size were ground and polished to 1-2 mmthickness to simulate fusion formed glass. These squares were placed ina 73 wt. % Li₂SO₄, 27% K₂SO₄ (close to eutectic composition) salt bathat 700° C. for roughly 2.8 hours to enable complete or nearly completeLi+ for Na+ ion exchange. The squares were cooled and rinsed with water.One crack was observed, but the square remained transparent. Little tono evidence of stress was seen in a polarimeter.

About one half of the square was then placed in a furnace and heattreated for 2 hours at 780° C. for nucleation and 4 hours at 900° C. forcrystal growth. The result was a translucent highly crystallineglass-ceramic whose composition was identified by x-ray diffraction(XRD) analysis as containing, a lithium aluminosilicate crystallinephase, particularly, a βspodumene solid solution phase, with a minorphase of aluminum titanate (Al₂TiO₅) being present.

Example 2

In a second example, a precursor glass from the complex SiO₂—Al₂O₃—Na₂Osystem and which was capable of being fusion formed was formed.Specifically, a complex sodium aluminosilicate glass with the followingbatched composition, in weight percent, was produced: 65.8% SiO₂, 19.0%Al₂O₃, 7.1% Na₂O, 1.1% MgO, 1.6% ZnO, 0.8% BaO, 0.3% SnO₂ and 4.3% TiO₂.A glass of this composition was subjected to the same glass forming, ionexchanging and ceramming conditions as described for Example 1.

Following the ion exchange process, the glass square was transparent andexhibited no cracks. After heat treatment the translucent glass-ceramicsubject to XRD analysis which revealed that the so-producedglass-ceramic was composed predominantly of a lithium aluminosilicatecrystalline phase, and particularly a “stuffed” β-quartz solid solutionphase, a phase with near-zero coefficient of thermal expansion, andincluded traces of β-spodumene solid solution and minor glass phase, aswell. Further heat treatment at a temperature of 1100° C. producedadditional crystallinity resulting in an opaque β-spodumene (also a lowCTE phase) glass-ceramic with a similar appearance to CorningWare®.

Example 3

In a third example, a precursor glass from the basic SiO₂—Al₂O₃—Na₂Osystem and which was capable of being down-drawn (particularly fusionformed) into a glass article/sheet was formed. Specifically, a simplesodium aluminosilicate glass with the following batched composition, inweight percent, was produced: 58.8% SiO₂, 21.5% Al₂O₃, 13.6% Na₂O, 0.3%SnO₂ and 4.3% TiO₂. As in example 1, this glass was batched, mixed andmelted in a platinum crucible at 1650° C. and thereafter annealed at650° C.

This glass was cut and polished into 1″ squares of ˜2 mm thickness andthen placed in a molten salt bath of a composition having 75 wt. %Li₂SO₄ and 25 wt. % Na₂SO₄ and held for two hours at a temperature of800° C. This time and temperature was sufficient to both allow Li+ forNa+ ion exchange and to allow internal nucleation and crystallization tooccur over the whole thickness of the glass; i.e., ion-exchange andceramming occurred simultaneously in the molten salt bath.

The resultant glass-ceramic article was white glass-ceramic andexhibited a glossy skin. XRD analysis revealed a composition comprisedof a lithium aluminosilicate (very low thermal expansion) as thepredominant crystalline phase, particularly a β-spodumene phase withminor rutile and glass phases. The fracture surface showed a finegrained crystallized texture throughout the body. The glass-ceramicappeared stronger than average in a hammer break test but broke intolarge fragments; i.e., the glass-ceramic article was not frangible.

Examples 4-7

Four additional glass-ceramic examples are listed in Table II below;particularly the precursor glass compositions (in weight percent) usedto produce the glass-ceramic. These glass-ceramics were produced fromprecursor glasses, as listed, in the same manner as that described abovefor Example 3. The precursor glasses were IOX'd and converted intoglass-ceramics in a single “IOX and nucleation/crystallization” stepsimilar manner to that described above for Example 3; the actual saltbath utilized for each of these examples was a molten salt bath of acomposition comprising 70 wt. % Li₂SO₄ and 30 wt. % Na₂SO₄, with theactual IOX times listed in Table II. Also listed in Table II is theliquidus temperature and viscosity at liquidus of each of the precursorglass.

In each of the four examples, 4-7, the resultant glass-ceramic articlewas a white glass-ceramic and XRD analysis revealed a compositioncomprised of lithium aluminosilicate (very low thermal expansion),particularly a β-spodumene solution phase, with a minor rutile phasebeing present.

TABLE II 4 5 6 7 SiO₂ 60.4 60.2 60.7 56.5 B₂O₃ 1.5 1.5 3.5 4.5 Al₂O₃22.1 21.0 18.0 19.5 Li₂O 1.5 1.5 1.0 1.1 Na₂O 10.9 11.8 12.3 13.4 MgO —— 0.5 0.5 TiO₂ 2.9 2.8 2.6 2.6 ZrO₂ 0.6 1.0 1.2 1.7 SnO₂ 0.2 0.2 0.2 0.2Liquidus temp. 1100 1050 950 1060 (° C.) Viscosity at 75,000 200,000300,000 Liquidus (Poise) IOX temp./time 850/3 750/4 750/4 850/16 (°C./Hrs.)

Examples 8-9

In another example, another precursor glass from the basicSiO₂—Al₂O₃—Na₂O system and capable of being down-drawn (particularlyfusion formed) into a glass article/sheet was formed. Specifically, asimple sodium aluminosilicate glass with the following batchedcomposition, in weight percent, was produced: 59.0% SiO₂, 21.6% Al₂O₃,13.6% Na₂O, 1.5% B₂O₃ and 4.3% TiO₂; titania being present at levelsrequired for eventual nucleation. A glass of this composition wassubjected to the same glass forming, and ceramming conditions asdescribed for Example 1, with the IOX conditions being varied asdescribed below.

Glass squares about 1″ in size were ground and polished to 1-2 mmthickness to simulate fusion formed glass. These squares were placed ina 70 wt. % Li₂SO₄, 30% K₂SO₄ (close to eutectic composition) salt bathat 850° C. In one embodiment, Example 8, the glass was IOX'd for aperiod of roughly 2 hours to achieve a varying Na₂O to Li₂O molar ratio(gradient) while in a second embodiment, Example 9, the glass was IOX'dfor a period of 8 hours to enable complete or nearly complete Li+ forNa+ ion exchange or no gradient or a flat alkali profile. In eachembodiment the squares were cooled and rinsed with water. FIGS. 1 and 2illustrates the respective Li₂O and Na₂O surface-to-depth concentrationsfor the Examples 8 and 9 above. As can be seen in the FIG. 1 plot theso-formed glass/glass-ceramic article exhibits an alkali gradient withthe Na varying from about 10% at the surface to about 5% in the bulk,while the Li₂O varies from about 1-2% on the surface to about a maximumof 9% in the bulk. As can be seen in the FIG. 2 plot, the so-formedglass/glass-ceramic article exhibits an flat alkali profile where theNa₂O is essentially 0% and has been replaced by Li₂O exhibiting a rangebetween about 9-11%.

In both embodiments the result was a translucent highly crystallineglass-ceramic whose composition was identified by x-ray diffraction(XRD) analysis as containing a lithium aluminosilicate crystallinephase, particularly a β-spodumene solid solution phase and minor phaseof rutile. Specifically, the glass-ceramic in both embodiments wasanalayzed to be comprised of the following composition: 63.5% SiO₂,23.2% Al₂O₃, 7.1% Li₂O, 1.6% B₂O₃ and 4.6% TiO₂.

Example 10

One final example, involved a utilizing a precursor glass from theSiO₂—Al₂O₃—Na₂O system, and capable of being down-drawn (particularlyfusion formed) into a glass article/sheet; in this example thecomposition was one from which a lithium metasilicate crystal phasewould be produced in the glass-ceramic form. Specifically, a sodiumaluminosilicate glass with the following batched composition, in molepercent, was produced: 74.8% SiO₂, 2.4% Al₂O₃ 19.6% Na₂O, 2.5% K₂O 0.67%ZnO, 0.06% Ag and 0.006% Au.

A glass of this composition was subjected to the same glass forming, andceramming conditions as described for Example 1, with the IOX conditionsbeing varied as described below.

Glass squares about 1″ in size were ground and polished to 1-2 mmthickness to simulate fusion formed glass. These squares were placed ina 100% LiNO₃ salt bath at 500° C. for a period of 6 hours to enablecomplete or nearly complete Li+ for Na+ ion exchange. The squares werecooled and rinsed with water.

Prior to heat-treatment the Example 10 sample was exposed to UV lightfor a period for 4 min, to produce precursor silver nuclei. Then, as inthe previous embodiments the sample was then placed in a furnace andheat treated in the following manner to achieve nucleation and crystalgrowth; 3-4° C./min ramp to approximately 550° C. followed by anapproximate ½ hold, approximately 2° C./min ramp to 600° C. followed bya an approximate 1 hour hold. The result was a opaque, 20% crystallineglass-ceramic whose composition was identified by x-ray diffraction(XRD) analysis as containing a lithium metasilicate crystalline phase.FIG. 3 is a SEM micrograph illustrating the Example 9 glass-ceramicLi-metasilicate crystal phase.

Various modifications and variations can be made to the materials,methods, and articles described herein. Other aspects of the materials,methods, and articles described herein will be apparent fromconsideration of the specification and practice of the materials,methods, and articles disclosed herein. It is intended that thespecification and examples be considered as exemplary.

We claim:
 1. A translucent or opaque glass-ceramic comprising: asilicate crystalline phase, and a composition, in weight percent on anoxide basis, 40-80% SiO₂, 2-30% Al₂O₃, 2-10% Li₂O, 0-8% TiO₂, 0-3% ZrO₂,and greater than 0% Na₂O, wherein a molar ratio of any one or more ofNa₂O/Al₂O₃+B₂O₃ and Li₂O+Na₂O/Al₂O₃+B₂O₃ is greater than about 0.8,wherein the glass-ceramic is made from a glass that exhibits a liquidusviscosity of greater than about 75,000 poise, and wherein a molar ratioof Na₂O to Li₂O increases from a surface to a bulk of the glass-ceramic.2. The glass-ceramic of claim 1, wherein the composition comprises anyone or more of: from greater than about 0% up to about 3% Na₂O, 0-2%SnO₂, 0-7% B₂O₃, 0-4% MgO, 0-12% ZnO, 0-8% BaO, 0-3% CaO, 0-6% SrO, 0-4%K₂O, 0-1.0% Sb₂O₃, 0-0.25% Ag, 0-0.25% CeO₂, and 0-0.01% Au.
 3. Theglass-ceramic of claim 1, wherein the composition is within theSiO₂—R₂O₃—Li₂O/Na₂O—TiO₂ system, and wherein R₂ comprises any one of Band Al.
 4. The glass-ceramic of claim 1, wherein the glass-ceramicarticle is formed via down drawn glass process.
 5. The glass-ceramic ofclaim 1, wherein the source of the Li in the glass-ceramic structure isprovided through an ion exchange process.
 6. The glass-ceramic of claim1, wherein the silicate crystal phase comprises a lithiumaluminosilicate phase of any one or more of β-spodumene and β-quartzsolid solution.
 7. The glass-ceramic of claim 1, wherein the silicatecrystal phase comprises any one or more of lithium metasilicate andlithium disilicate.
 8. The glass-ceramic of claim 1, wherein thecomposition further includes the combination of TiO₂+ZrO₂+SnO₂ in anamount of at least 2.0 mol %.
 9. The glass-ceramic of claim 8, whereinthe composition comprises, in weight percent on an oxide basis, 55-70%SiO₂, 17-23% Al₂O₃, 0-5% B₂O₃, 2.5-7% Li₂O, 0-3% ZrO₂, 0-1% SnO₂, 0-3%MgO, 0-6% ZnO, up to 2% Na₂O, and 2-4.5% TiO₂.
 10. The glass-ceramic ofclaim 1, wherein said composition further comprises a transition metalion selected from the group consisting of Co²⁺, Cr³⁺, Cu¹⁺, Sn⁴⁺, Mn⁴⁺,Sb³⁺, Fe³⁺, In³⁺, Bi³⁺, Ni²⁺, V³⁺, and Ta⁵⁺.
 11. The glass-ceramic ofclaim 1, further comprising a compressive stress layer extending fromthe surface to the bulk.